1. Field of the Invention
The present invention relates to an electrostatic latent image bearer (hereinafter referred to as an electrophotographic photoreceptor or a photoreceptor) for use in copiers, electrostatic printings, electrostatic recording, etc., and to an image forming method, an image forming apparatus and a process cartridge using the electrostatic latent image bearer.
2. Discussion of the Background
In image forming apparatuses such as a copier, a printer and a facsimile using an electrophotographic method, writing light modulated with image data is irradiated to a uniformly charged photoreceptor to form an electrostatic latent image thereon; and an image developer provides a toner to the electrostatic latent image to form a toner image thereon. After the image forming apparatus transfers the toner image onto a transfer sheet (recording paper) with a transferer, fixes the toner image on the transfer sheet upon application of heat and pressure with a fixer and collects the toner remaining on the photoreceptor with a cleaner such as a cleaning blade.
In such image forming apparatuses using electrophotographic methods, organic photoreceptors including organic photoconductive materials are most widely used. The organic photoreceptors have more advantages than other photoreceptors because materials in compliance with various irradiating light sources from visible light to infrared are easy to develop, materials free from environment pollution can be selected, the production cost thereof is low, etc. However, the organic photoreceptor has low mechanical strength and the photosensitive layer thereof is abraded after used for long periods. When the photosensitive layer is abraded in a specific amount, the electrical properties of the photoreceptor vary, resulting in occasional failure of proper image forming process. The photoreceptor is abraded in all parts contacting to other image forming units such as an image developer and a transferer.
Various suggestions are made to improve lives of photoreceptors by reducing the abrasion of photosensitive layers.
Japanese Patent No. 3258397 discloses a surface protective layer wherein a hardening silicone resin including colloidal silica is used. Although the abrasion resistance thereof is improved, foggy images and blurred images tend to be produced due to repeated use. In addition, the durability thereof is still insufficient for long-life photoreceptors recently required.
Japanese Patent No. 3640444 discloses polysiloxane positive-hole transport material formed by hardening silicon positive-hole transport material, which a silyl group having a hydrolyzable group is introduced to, and a polysiloxane resin as a protection material for the surface of a photoreceptor. Japanese Patent No. 3267519 discloses a photoreceptor having a surface layer including a resin formed by hardening a hardening organic silicon polymer and an organic silicon-modified positive-hole transport material.
However, blurred images tend to be produced and occurrence thereof needs to be prevented by a drum heater, etc., resulting in larger apparatus and higher costs thereof. In addition, the residual potential of irradiated parts thereof does not sufficiently decreases, resulting in deterioration of image density in a low-potential developing process controlling the charge potential.
Japanese Laid-Open Patent Publication No. 2000-171990 discloses a method of hardening a hardening siloxane resin having a charge transportability imparting group in the form of a three-dimensional network. The coated layer occasionally cracks due to the volume contraction, particularly with an inexpensive marketed coating agent easy to use. In addition, the residual potential of irradiated parts thereof depends on the layer thickness, resulting in deterioration of image density in a low-potential developing process. When the charge transportability imparting group is increased, the strength of the coated layer deteriorates, resulting occasional insufficient durability. Further, blurred images are occasionally produced.
Japanese Laid-Open Patent Publication No. 2000-171990 discloses a method of using a urethane resin in the protective layer of a photoreceptor to improve the abrasion resistance. However, although a urethane resin used in the protective layer considerably improves the abrasion resistance, the protective layer does not have sufficient adhesiveness to a binder resin, such as a polycarbonate resin, used in the photosensitive layer thereunder. Therefore, the protective layer occasionally peels therefrom at the end of a photoreceptor or because of a damage on the surface of the protective layer due to a carrier or a paper dust. At the part where the protective layer is peeled from the photosensitive layer, the photosensitive layer is exposed, and which has different chargeability and light transmittance, resulting in abnormal images such as unevenly colored images.
In addition, when the protective layer becomes thinner due to abrasion, the protective layer more easily peels and quickly disappears. When the thickness thereof is more thickened, the residual potential of the irradiated part increases, resulting in insufficient quality halftone images and low image density in negative-positive developing digital image forming apparatuses.
Further, recently, a spherical polymerized toner is put onto practical use due to requests for higher-quality images. However, the polymerized toner is known to typically be difficult to clean with a cleaning blade made of a urethane rubber. Therefore, the contact pressure is increased to improve the removability of the toner, but which not only accelerates the abrasion of a photoreceptor but also encourages the peeling of the protective layer. Accordingly, the image forming apparatus using the polymerized toner needs a photoreceptor which has a protective layer having higher abrasion resistance and no peeling.
Because of these reasons, a need exists for a highly-durable electrostatic latent image bearer having good electrophotographic image formability and capability of forming stable images for long periods in addition to high abrasion resistance and good adhesiveness of the protective layer to the photosensitive layer.